Element that oscillates or detects terahertz waves

ABSTRACT

An element which oscillates or detects terahertz waves includes a resonance unit including a differential negative resistance element, a first conductor, a second conductor, and a dielectric body, a bias circuit configured to supply a bias voltage to the differential negative resistance element, and a line configured to connect the resonance unit and the bias circuit to each other. The differential negative resistance element and the dielectric body are disposed between the first and second conductors. The line is a low impedance line in a frequency f LC  of resonance caused by inductance of the line and capacitance of the resonance unit using an absolute value of a differential negative resistance of the differential negative resistance element as a reference.

TECHNICAL FIELD

The present invention relates to an element which oscillates or detectsterahertz waves.

BACKGROUND ART

Oscillators including a resonator integrated in a negative resistanceelement have been widely used as current injection light sources whichgenerate electromagnetic waves (hereinafter referred to as “terahertzwaves”) in a frequency domain from a millimeter waveband to a terahertzwaveband (from 30 GHz inclusive to 30 THz inclusive). PTL 1 discloses anoscillator for terahertz waves which includes a double barrier resonanttunneling diode (hereinafter simply referred to as an “RTD” whereappropriate) serving as a negative resistance element and a micro-stripresonator which are integrated in the same substrate.

Oscillators using a negative resistance element usually generateparasitic oscillation caused by a bias circuit which includes a powersource and wiring which are used to control a bias voltage of thenegative resistance element. The parasitic oscillation generated in alower frequency band other than a desired frequency deterioratesoscillation output in the desired frequency.

To suppress the parasitic oscillation, assuming that a wavelength of aterahertz wave oscillated by the oscillator is denoted by λ_(THz) and anoscillation frequency is denoted by f_(THz), impedance on a bias circuitside is reduced in a frequency domain equal to or larger than DC andsmaller than f_(THz). To address this problem, a method for arranging acircuit including a resistance and a capacitance in a position withinλ_(THz)/4 on a power source side viewed from the RTD has been proposed.

As a device which realizes this method, NPL 1 discloses an oscillatoremploying a slot resonator. In NPL 1, as illustrated in FIG. 11, arectifying diode 15 is disposed in a position within λ_(THz)/4 on apower source 16 side viewed from S-RTD 11. Note that a resistance 17 isobtained by adding an internal resistance of the power source 16 and aresistance of a connecting wire to each other.

The method employed in NPL 1 is employed only for slot resonators, andtherefore, it is difficult to employ the method in oscillators employinga microstrip resonator, such as a patch antenna disclosed in PTL 1. Thisis because, if the resonator is a patch antenna, for example, a regionwithin λ_(THz)/4 on a bias circuit side viewed from a negativeresistance element includes the patch antenna or peripherals of thepatch antenna which are arranged close to the patch antenna. Therefore,in microstrip resonators, it is difficult to arrange a circuit such thatthe circuit does not interfere with a resonator.

Furthermore, since a line structure for controlling a bias voltage ofthe negative resistance element is arranged close to the resonator,parasitic oscillation of comparatively high frequencies caused by thestructure is required to be reduced.

CITATION LIST Patent Literature

PTL 1 U.S. Patent Application Publication No. 2006/0055476

Non Patent Literature

NPL 1 IEEE Electron Device Letters, Vol 18, 218 (1997)

SUMMARY OF INVENTION

The present invention provides an element which oscillates or detectsterahertz waves and which includes a resonance unit including adifferential negative resistance element, a first conductor, a secondconductor, and a dielectric body, a bias circuit configured to supply abias voltage to the differential negative resistance element, and a lineconfigured to connect the resonance unit and the bias circuit to eachother. The differential negative resistance element and the dielectricbody are disposed between the first and second conductors, and the lineis a low impedance line in a frequency f_(LC) of resonance caused byinductance of the line and capacitance of the resonance unit using anabsolute value of a differential negative resistance of the differentialnegative resistance element as a reference.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a perspective view of a configuration of an element accordingto an embodiment.

FIG. 1B is a sectional view of the element according to the embodimenttaken along a line IB-IB of FIG. 1A.

FIG. 1C is a sectional view of the element according to the embodimenttaken along a line IC-IC of FIG. 1A.

FIG. 2 is a diagram illustrating an admittance characteristic of theelement according to the embodiment.

FIG. 3A is a perspective view of a configuration of an element accordingto a first example.

FIG. 3B is a top view of a line of the element according to the firstexample.

FIG. 4A is a perspective view of a configuration of an element of afirst modification according to the first example.

FIG. 4B is a sectional view of the element of the first modificationaccording to the first example taken along a line IVB-IVB of FIG. 4A.

FIG. 5A is a perspective view of a configuration of an element of asecond modification according to the first example.

FIG. 5B is a sectional view of the element of the second modificationaccording to the first example taken along a line VB-VB of FIG. 5A.

FIG. 5C is a sectional view of the element of the second modificationaccording to the first example taken along a line VC-VC of FIG. 5A.

FIG. 6 is a diagram illustrating an admittance characteristic of theelement of the second modification according to a second example.

FIG. 7A is a perspective view of a configuration of an element of athird modification according to the second example.

FIG. 7B is a sectional view of the element of the third modificationaccording to the second example taken along a line VIIB-VIIB of FIG. 7A.

FIG. 7C is a sectional view of the element of the third modificationaccording to the second example taken along a line VIIC-VIIC of FIG. 7A.

FIG. 8 is a diagram illustrating an admittance characteristic of theelement of the third modification according to the second example.

FIG. 9A is a diagram illustrating dependence of a thickness of adielectric body of a real part of admittance of the element according tothe embodiment.

FIG. 9B is a diagram illustrating dependence of the thickness of thedielectric body of an imaginary part of the admittance of the elementaccording to the embodiment.

FIG. 10A is a diagram illustrating dependence of a length of a line ofthe real part of the admittance of the element according to theembodiment.

FIG. 10B is a diagram illustrating dependence of the length of the lineof the imaginary part of the admittance of the element according to theembodiment.

FIG. 11 is a diagram illustrating a configuration of an oscillatordisclosed in the related art.

DESCRIPTION OF EMBODIMENTS

An element 100 according to an embodiment will be described. The element100 is an oscillator which oscillates electromagnetic waves of anoscillation frequency (a resonance frequency) f_(THz). FIG. 1A is aperspective view of the element 100 according to this embodiment, FIG.1B is a sectional view of the element 100 taken along a line IB-IB ofFIG. 1A, and FIG. 1C is a sectional view of the element 100 taken alonga line IC-IC of FIG. 1A. Note that the element 100 is referred to as an“oscillator 100” hereinafter.

A configuration of the oscillator 100 will now be described. Theoscillator 100 includes a resonance unit 102, a line 108, and a biascircuit 120. The resonance unit 102 includes a resonator of a terahertzband including a patch conductor (first conductor) 103, a groundingconductor (second conductor) 104, and a first dielectric body 105 a anda differential negative resistance element 101 (hereinafter referred toas an “element 101”).

In the element 101, a region in which a current is reduced when avoltage is increased, that is, a region having negative resistance (adifferential negative resistance region), appears as a current-voltagecharacteristic. A high frequency element, such as an RTD, an Esakidiode, Gunn diode, or a transistor having one terminated end, istypically used as a preferred element 101. Alternatively, a TUNNETTdiode, an IMPATT diode, a heterojunction bipolar transistor (HBT), acompound semiconductor FET, or a high-electron-mobility transistor(HEMT) may be used. Furthermore, a differential negative resister of aJosephson device employing a superconductor may be used. In thisembodiment, a case where a resonant tunneling diode (RTD) which is atypical differential negative resistance element operating in aterahertz band is used as the element 101 will be described as anexample.

The resonance unit 102 which is an active antenna includes a resonatorand the element 101 integrated thereon. The resonator of a terahertzwave includes the patch conductor 103, the grounding conductor 104, andthe dielectric body 105 a disposed between the patch conductor 103 andthe grounding conductor 104. The configuration in which the firstdielectric body 105 a is sandwiched between the patch conductor 103 andthe grounding conductor 104 is widely used in microstrip resonatorsusing a microstrip line having a finite length or the like. In thisembodiment, a patch antenna is used as the resonator for terahertzwaves.

The patch antenna is set such that a width of the patch conductor 103 ina direction of a line IB-IB corresponds to λ/2 resonator. The element101 is disposed between the patch conductor 103 and the groundingconductor 104. In this embodiment, the resonance unit 102 is configuredsuch that the element 101 and the resonator of a terahertz band, such asthe patch antenna, are integrated.

Here, in this specification, the term “dielectric body” is a materialwhich has a dielectric property superior to conductivity and whichserves as an insulating body or a high resistive body which blockselectricity relative to direct voltage. Typically, a material having aresistivity of 1 kΩ·m or more is preferably used. Specifically, plastic,ceramic, silicon oxide, silicon nitride, or the like is preferably used.

The resonance unit 102 is an active antenna including the patch antennaand the differential negative resistance element 101 integrated thereon.Therefore, the oscillation frequency f_(THz) defined by the resonanceunit 102 of the oscillator 100 is determined as a resonance frequency ofa whole parallel resonance circuit configured such that reactance of thepatch antenna and reactance of the differential negative resistanceelement 101 are combined with each other. That is, the oscillator 100oscillates terahertz waves of the oscillation frequency f_(THz).

Specifically, according to an equivalent circuit of an RTD oscillatordisclosed in Jpn. J. Appl. Phys., Vol. 47, No. 6, 4375 (2008), for aresonance circuit configured such that an RTD and admittance (Y_(RTD)and Y_(ANT)) of an antenna are combined with each other, a frequencywhich satisfies two conditions, that is, an amplitude conditionrepresented by Expression (1) and a phase condition represented byExpression (2), is determined as the oscillation frequency f_(THz).Here, Re[Y_(RTD)] represents admittance of the element 101 and has anegative value.Re[Y _(RTD) ]+Re[Y _(ANT)]≤0  (1)Im[Y _(RTD) ]+Im[Y _(ANT)]=0  (2)

The bias circuit 120 which supplies a bias voltage to the element 101includes a resistance 110 connected in parallel to the element 101, acapacitance 109 connected in parallel to the resistance 110, a powersource 112, and a line 111. The line 111 constantly has a parasiticinductance component, and therefore, the line 111 is represented as aninductance in FIG. 1A. The power source 112 supplies current requiredfor driving the element 101 and controls the bias voltage. The biasvoltage is typically selected from the differential negative resistanceregion of the element 101.

The line 108 is a distributed constant line. The bias voltage issupplied from the bias circuit 120 through the line 108 to the element101. The line 108 of this embodiment is a microstrip line. The line 108includes a strip conductor (third conductor) 106, the groundingconductor (fourth conductor) 104, a second dielectric body 105 b, and afifth conductor 107, and is configured such that the second dielectricbody 105 b is sandwiched between the strip conductor 106 and thegrounding conductor 104. The strip conductor 106 and the patch conductor103 are electrically connected to each other through the fifth conductor107.

A thickness of the second dielectric body 105 b is smaller than that ofthe first dielectric body 105 a. Therefore, the fifth conductor 107serves as a plug for covering a gap (difference in height) between thefirst dielectric body 105 a and the second dielectric body 105 b andelectrically and physically connecting the patch conductor 103 and thestrip conductor 106 to each other. In this way, the resonance unit 102and the line 108 are coupled with each other by DC coupling. Note that,although the second conductor of the resonance unit 102 and the fourthconductor of the line 108 both correspond to the grounding conductor 104in the same layer in this embodiment, different conductors may beemployed.

The resistance 110 and the capacitance 109 of the bias circuit 120suppress parasitic oscillation of a resonance frequency f_(sp)(f_(sp)<f_(LC)<f_(THz), and typically, in a frequency band from DC to 10GHz) which is a comparatively low frequency caused by the bias circuit120. Here, the frequency f_(LC) is a frequency of an LC resonance of aninductance L of the line 108 and a capacitance C_(ANT) of the resonanceunit 102 having the element 101 and the patch antenna. The frequencyf_(LC) will be described in detail hereinafter.

A value of the resistance 110 is preferably equal to or slightly smallerthan an absolute value of the differential negative resistance in thedifferential negative resistance region of the element 101. Theresistance 110 is located in a position away from the element 101 by adistance d₂. Then, mainly in a wavelength band of 4×d₂ or more, aportion of the bias circuit 120 on an outside relative to the resistance110 has a low impedance relative to the element 101, that is, a lowimpedance when using the absolute value of the differential negativeresistance of the element 101 as a reference. That is, the resistance110 is preferably set such that the resistance 110 has a low impedancerelative to the element 101 in a frequency band equal to or smaller thanthe frequency f_(SP) (f_(SP)<f_(LC)<f_(THz)).

As with the resistance 110, the capacitance 109 is set such that theimpedance of the capacitance 109 is equal to or slightly smaller thanthe absolute value of the differential negative resistance of theelement 101. In general, the capacitance 109 is preferably large and isapproximately several tens of pF in this embodiment. The capacitance 109is a decoupling capacitance directly coupled with the microstrip lineserving as the line 108, and may have an MIM (Metal-insulator-Metal)structure including a substrate (not illustrated) shared by theresonance unit 102, for example.

In configurations in the related art, an LC resonance of a frequencyf_(LC)(f_(LC)≈1/(2π√(LC_(ANT))), f_(SP)<f_(LC)<f_(THz)) caused by aninductance L of a line and a capacitance C_(ANT) of a differentialnegative resistance element and the resonance unit may be formed. Thefrequency f_(LC) is determined mainly using capacitance of adifferential negative resistance element, a length and a width of a line(the microstrip line in this embodiment), an area of a patch antenna, athickness of a dielectric body, and the like, and is typically in arange from 10 GHz inclusive to 500 GHz inclusive.

Due to a configuration of a patch antenna, it is difficult to directlyconnect a bias circuit to a resonance unit without interference with aresonance electric field of an oscillation frequency f_(THz). Therefore,to supply a bias voltage to an element, a bias circuit and a resonanceunit are connected to each other through a line serving as a powersupply line, and accordingly, parasitic oscillation of a frequencyf_(LC) may be generated due to a gain of the element 101.

In the oscillator 100, the resonance unit 102 including the element 101and the patch antenna is connected to the bias circuit 120 including theresistance 110 and the capacitance 109 through the line 108 serving asthe distributed constant line. Accordingly, the line 108 serving as thedistributed constant line is used to connect the capacitance 109 whichis the decoupling capacitance of the bias circuit 120 and the resonanceunit 102 including the patch antenna to each other. The oscillator 100of this embodiment is an active patch antenna which is configured suchthat the element 101 and the first dielectric body 105 a are sandwichedbetween the two conductors, that is, the patch conductor 103 and thegrounding conductor 104. The resistance 110 of the bias circuit 120which is a shunt resistance and the capacitance 109 which is thedecoupling capacitance are connected to the element 101 and the powersource 112 in parallel. The line 108 which is the microstrip line isconnected between the patch antenna of the resonance unit 102 and thecapacitance 109.

In the oscillator 100, the line (microstrip line) 108 has a lowimpedance relative to the element 101 in a frequency band in thevicinity of the frequency f_(LC) of the LC resonance. The configurationin which the line 108 has a low impedance relative to the element 101means a configuration in which the line 108 is a low impedance lineusing the absolute value of the differential negative resistance of theelement 101 as a reference.

Here, the low impedance line using the absolute value of thedifferential negative resistance of the element 101 as a reference meansa line having a comparatively low impedance using the absolute value ofthe differential negative resistance of the element 101 as thereference. The line having the comparatively low impedance typicallymeans a line having an impedance equal to or smaller than a value tentimes as large as the absolute value of the differential negativeresistance of the differential negative resistance element or preferablyequal to or smaller than the absolute value of the differential negativeresistance. In general, the absolute value of the differential negativeresistance of the differential negative resistance element isapproximately equal to or larger than 0.1Ω to equal to or smaller than100Ω, and therefore, the impedance of the low impedance line is set in arange from approximately 1Ω inclusive to approximately 1000Ω inclusive.

To set the line 108 having the low impedance using the absolute value ofthe differential negative resistance of the element 101 serving as thereference, a thickness of the line 108 is controlled, for example, sothat a characteristic impedance of the line 108 is set to a value tentimes as large as the absolute value of the differential negativeresistance of the element 101 or less. More preferably, thecharacteristic impedance of the line 108 is set to a value equal to orsmaller than the absolute value of the differential negative resistanceof the element 101. Furthermore, an absolute value (|1/Re[Y_(RTD)]|) ofa real part of the impedance which is a gain may be used taking afrequency characteristic of the element 101 into consideration. In thiscase, a line having a characteristic impedance equal to or smaller thana value which is ten times as large as an absolute value(|1/Re[Y_(RTD)]|) of the real part of the impedance of the element 101,or preferably, a line having a characteristic impedance equal to orsmaller than the absolute value (|1/Re[Y_(RTD)]|) of the real part ofthe impedance of the element 101 is used as the line 108.

With this configuration, loss of electromagnetic waves of frequencies inthe vicinity of the frequency f_(LC) is large in the line 108.Therefore, since the microstrip line serving as the line 108 is a lossyline, loss of the parasitic LC resonance caused by the line 108 isincreased. Accordingly, oscillation of the LC resonance may be blockedor reduced.

Here, when a length of the line 108 is denoted by d₁ and a distance fromthe element 101 to the resistance 110 is denoted by d₂, a frequency bandin the vicinity of the frequency f_(LC) corresponds to a wavelength bandequal to or larger than 4×d₁ and equal to or smaller than 4×d₂ when thefrequency band is converted into the wavelength. The wavelength band isdetermined in accordance with arrangement and structures of the line 108and the resistance 110, and is typically a range from several GHzinclusive to 500 GHz inclusive.

Note that the configuration represented by the expression “the line 108is a low impedance line using the absolute value of the differentialnegative resistance of the element 101 as a reference” in thisspecification may be easily designed taking the characteristic impedanceof the line 108 into consideration. Specifically, the line 108 isdesigned such that the characteristic impedance of the line 108 is equalto or smaller than a value which is ten times as large as the absolutevalue of the differential negative resistance of the element 101 orpreferably, the characteristic impedance of the line 108 is equal to orsmaller than the absolute value of the differential negative resistanceof the element 101. In this case, the line 108 is a low impedance linerelative to the absolute value of the differential negative resistanceof the differential negative resistance element 101 serving as areference.

When the impedance of the line 108 relative to the element 101 in thefrequency f_(LC) is equal to or smaller than the value which is as largeas ten times the absolute value of the differential negative resistance,loss of the gain of the element 101 caused by the line 108 is notnegligible, and therefore, the oscillation of the LC resonance may besuppressed. In particular, the lower the impedance of the line 108relative to the element 101 in the frequency f_(LC) is, the larger theloss of the electromagnetic waves of this frequency band of the line 108is, and accordingly, the oscillation of the LC resonance is efficientlysuppressed. When the impedance of the line 108 relative to the element101 in the frequency f_(LC) is equal to or smaller than the absolutevalue of the differential negative resistance, the loss is larger thanthe gain of the element 101, and accordingly, oscillation of the LCresonance may be blocked.

Furthermore, when the impedance of the line 108 is designed to be lowrelative to the element 101, a structure and a configuration whichattain the effect described above may be realized. Furthermore, as astructure in which the impedance is low in the line 108 relative to theelement 101 is located closer to the element 101 or the resonance unit102, the oscillation of the LC resonance is more effectively suppressed.Specifically, the structure is preferably located in a distance equal toor smaller than λ_(THz) from the element 101. Note that “λ_(THz)” is awavelength of a terahertz wave of the oscillation frequency f_(THz).

With this configuration, due to the effect of the loss of the line 108,Expression (1) is not satisfied in a low frequency domain smaller thanf_(THz) including frequencies f_(sp) and f_(LC), and the followingexpression is satisfied: Re[Y_(RTD)]+Re[Y_(ANT)]>0. On the other hand,in a frequency domain in the vicinity of the oscillation frequencyf_(THz), a structure which satisfies Expression (1) may be realized.Here, “Re[Y_(RTD)]” represents a negative value and approximates aninverse number of the differential negative resistance of thedifferential negative resistance element 101 in DC. Accordingly, theoscillation of the LC resonance may be suppressed, and terahertz wavesof the oscillation frequency f_(THz) may be obtained.

Loss of a distributed constant line, such as the line 108, includesconductor loss caused by a skin effect and dielectric loss caused by adielectric tangent (tan δ). On an equivalent circuit of a distributedconstant line, the conductor loss and the dielectric loss arerepresented by a resistance R connected to an inductance L in series anda leakage conductance G connected to a capacitance in parallel,respectively. The leakage conductance G is represented by an equation“G=tan δ×ω×C”, and therefore, as a frequency becomes higher, such asfrequencies of terahertz waves, the dielectric loss is not negligible.Furthermore, the dielectric loss corresponds to an increasing functionof the dielectric tangent tan δ and the capacitance C of the line 108.

Accordingly, when a material and a structure of the line 108 areselected and the dielectric tangent tan δ and the capacitance C of theline 108 are appropriately selected, the leakage conductance G ofelectromagnetic waves in the vicinity of the frequency f_(LC) of theline 108 may be increased and the dielectric loss in the vicinity of thefrequency f_(LC) may be increased. Specifically, the dielectric tangenttan δ and the capacitance C of the line 108 are selected such that thedielectric loss becomes equal to or larger than one-tenth of an absolutevalue of an inverse number of the differential negative resistance ofthe element 101, or more preferably, equal to or larger than theabsolute value of the inverse number of the differential negativeresistance. With this configuration, the oscillation in the vicinity ofthe frequency f_(LC) is efficiently suppressed.

For example, when a thickness of the dielectric body 105 b issufficiently small, the capacitance C of the microstrip line 108 isincreased, and accordingly, the dielectric loss is increased in thevicinity of the frequency f_(LC) and the line 108 becomes a lowimpedance line relative to the absolute value of the differentialnegative resistance of the element 101 serving as a reference.Specifically, although depending on a type of the dielectric body 105 b,typically, when the thickness of the dielectric body 105 b is equal toor larger than 0.001 μm and equal to or smaller than 1 μm, thecharacteristic impedance of the microstrip line 108 in the vicinity ofthe frequency f_(LC) is equal to or larger than 1Ω and equal to orsmaller than 100Ω. Although depending on a structure, the absolute valueof the differential negative resistance of the element 101 isapproximately equal to or larger than 1Ω and equal to or smaller than100Ω.

Therefore, the characteristic impedance may be set to a value equal toor smaller than a value ten times as large as the absolute value of thedifferential negative resistance of the element 101 or equal to orsmaller than the absolute value of the differential negative resistanceby controlling the thickness of the line 108. In this case, theimpedance of the line 108 relative to the element 101 is equal to orsmaller than a value ten times as large as the absolute value of thedifferential negative resistance or equal to or smaller than theabsolute value of the differential negative resistance. As a result, thedielectric loss of the microstrip line 108 in the vicinity of thefrequency f_(LC) is increased, and accordingly, the oscillation of theLC resonance may be suppressed or blocked.

Since the thickness of the second dielectric body 105 b is smaller thanthe thickness of the first dielectric body 105 a of the resonance unit102, the patch antenna serving as a resonator/radiator for terahertzwaves may increase only loss of the line 108 while maintaining low lossand high radiation efficiency. Specifically, electromagnetic waves ofthe oscillation frequency f_(THz) may maintain the low loss and the highradiation efficiency in the resonance unit 102 and increase loss of theelectric waves in the frequency f_(LC) in the microstrip line 108.

FIG. 9A is a graph obtained by plotting a result of analysis ofdependence of a thickness of the dielectric body 105 b of the real partof the admittance of the oscillator 100. FIG. 9B is a graph obtained byplotting a result of analysis of dependence of the thickness of thedielectric body 105 b of the imaginary part of the admittance of theoscillator 100. As illustrated in FIGS. 9A and 9B, silicon nitridefrequently used in semiconductor fabrication is used as the seconddielectric body 105 b. In a case of a general oscillator including thesecond dielectric body 105 b of 3 μm, admittance of a value equal to orsmaller than 10 mS is obtained in the vicinity of the frequencyf_(LC)(≈0.08 THz), that is, low loss (high impedance) configuration isobtained. On the other hand, in the oscillator 100, when the filmthickness is made thin in a range from 1 μm inclusive to 0.1 μminclusive, change of admittance in the vicinity of the oscillationfrequency f_(THz) is minimum and admittance in the vicinity of thefrequency f_(LC) may be arbitrarily controlled from 10 mS to 100 mS ormore.

In this way, the thickness of the second dielectric body 105 b of theline 108 is set such that the thickness of the second dielectric body105 b is smaller than that of the first dielectric body 105 a of theresonance unit 102. By this, a difference between equivalent refractiveindices of the line 108 and the resonance unit 102 becomes large, andaccordingly, impedance mismatching occurs in a portion in which theresonance unit 102 and the line 108 are connected to each other.Therefore, when the dielectric body 105 b becomes thin, the LC resonanceof the frequency f_(LC) caused by the capacitance of the resonance unit102 including the antenna and the inductance of the line 108 is dividedinto resonance caused by the capacitance of the resonance unit 102 andresonance caused by the inductance of the line 108. Consequently, tworesonance points having similar frequencies are generated in positionsbefore and after the frequency f_(LC), and low impedance bands aregenerated in positions before and after the frequency f_(LC).

Furthermore, when a material having a large dielectric tangent tan δ isused as the dielectric body 105 b, the dielectric loss of theelectromagnetic waves in the vicinity of the frequency f_(LC) may beincreased without changing a structure size of the line 108. Examples ofthe material having a large dielectric tangent tan δ in the frequencyf_(LC) include silicon nitride, polyaramide, polyethylene terephthalate,PMMA, ABS, and polycarbonate. Furthermore, a substance obtained bydistributing fine powder of a metal filler or carbon graphite on asynthetic resin in which a magnitude of dielectric loss has electricfield intensity dependence, such as styrol or epoxy, may be used.Moreover, a metamaterial structure having a large dielectric tangent tanδ in the frequency f_(LC) may be used.

Here, the length d₁ of the line 108 is preferably large as long as aconductor resistance does not become considerably large, and ispreferably equal to or larger than λ_(THz)/4. Note that “λ_(THz)”represents a wavelength of terahertz waves of the oscillation frequencyf_(THz).

FIG. 10A is a graph obtained by plotting a result of analysis ofdependence of a length of the line 108 of the real part of theadmittance of the oscillator 100. FIG. 10B is a graph obtained byplotting a result of analysis of dependence of the length of the line108 of an imaginary part of the admittance of the oscillator 100. FIGS.10A and 10B represent the real part and the imaginary part of theadmittance of the oscillator 100 obtained when the length of the line108 is set to 100 μm, 200 μm, 300 μm, 400 μm, and 500 μm. When “λ_(THz)”represents a wavelength of a terahertz wave in the frequency f_(THz),100 μm corresponds to λ_(THz)/4. A long strip conductor 106 causes anincreased inductance L and the frequency f_(LC) of the LC resonance isshifted toward a low frequency side. Therefore, influence of loss in thevicinity of the frequency f_(LC) to an electric field of terahertz wavesof the oscillation frequency f_(THz) in the resonance unit 102 issuppressed. On the other hand, a short strip conductor 106 causesreduced inductance L and the frequency f_(LC) of the LC resonance isshifted toward a high frequency side.

As a result, the influence of the oscillation Frequency f_(THz) to theelectric field of terahertz waves of the oscillation frequency f_(THz)may not be negligible in terms of a distance and a frequency, andaccordingly, this causes loss of terahertz waves of the oscillationfrequency f_(THz) and multi-mode. This is because, the length of theline 108 and a frequency band of low impedance in the vicinity of thefrequency f_(LC) become large and the frequency f_(LC) shifts on a lowfrequency side. Therefore, the length of the line 108 is preferablylarge, and is more preferably equal to or larger than λ_(THz)/4. Notethat, since a large length of the line 108 causes series resistance, theline 108 is appropriately designed taking the relationship between theseries resistance and the impedance in the vicinity of the frequencyf_(LC) into consideration.

The conductor 107 and the line 108 preferably have widths which do notinterfere with the resonance electric field in the resonance unit 102,and the widths are preferably equal to or smaller than λ_(THz)/10, forexample. Furthermore, the conductor 107 and the line 108 are preferablydisposed in nodes of electric fields of terahertz waves of theoscillation frequency f_(THz) steadily existing in the resonance unit102. Here, the line 108 has higher impedance than the absolute value ofthe differential negative resistance of the element 101 in a frequencyband in the vicinity of the oscillation frequency f_(THz). Therefore,influence of the loss of electromagnetic waves in the vicinity of thefrequency f_(LC) to the electric field of the oscillation frequencyf_(THz) in the resonance unit 102 is suppressed.

Here, the expression “nodes of electric fields of terahertz waves of theoscillation frequency f_(THz) steadily existing in the resonance unit102” means regions corresponding to substantive nodes of the electricfields of terahertz waves of the oscillation frequency f_(THz) steadilyexisting in the resonance unit 102. Specifically, the expression “nodesof electric fields of terahertz waves of the oscillation frequencyf_(THz) steadily existing in the resonance unit 102” also means regionscorresponding to substantive nodes of the electric fields of terahertzwaves of the oscillation frequency f_(THz) steadily existing in thepatch antenna serving as a resonator for terahertz waves. Specifically,the expression “nodes of electric fields of terahertz waves of theoscillation frequency f_(THz) steadily existing in the resonance unit102” means regions in which electric field intensity f_(THz) ofterahertz waves of the unit 102 is lower than the maximum electric fieldintensity of the terahertz waves of the oscillation frequency f_(THz)steadily existing in the resonance unit 102 by approximately singledigit. Preferably, a position in which electric field intensity ofterahertz waves of the oscillation frequency f_(THz) is equal to orsmaller than 1/e² (“e” is a base of natural logarithm) of the maximumelectric field intensity of the terahertz waves of the oscillationfrequency f_(THz) steadily existing in the resonance unit 102 isemployed.

A result of analysis of an admittance characteristic of an oscillator200 according to a first concrete example of this embodiment will bedescribed with reference to FIG. 2. A configuration of the oscillator200 will be described in the first example hereinafter.

FIG. 2 is a graph illustrating a result of analysis of the admittancecharacteristic of the oscillator 200 and plotting a real partRe[Y_(RTD)] of admittance of an RTD 201 which is a differential negativeresistance element and a real part Re[Y_(ANT)] of admittance of a patchantenna. In the graph, an admittance Re[Y_(ANT with 208)] of a patchantenna including a line 208 which has low impedance in a frequency bandin the vicinity of a frequency f_(LC) in the first example describedhereinafter is also illustrated. Furthermore, an admittance Re[Y_(ANT)]of a patch antenna including a general microstrip line havingcomparatively high impedance as a line is also illustrated.

It is assumed that, in the oscillator 200, an oscillation frequencyf_(THz) which satisfies Expression (2) is 0.45 THz and a frequencyf_(LC) of an LC resonance is 0.08 THz. In a case of general microstriplines having comparatively high impedance, the oscillation condition ofExpression (1) is satisfied in terms of the frequency and the frequencyf_(THz) and the frequency f_(LC), and therefore, parasitic oscillationhaving LC resonance may be generated.

On the other hand, when the line (microstrip line) 208 which has lowimpedance in a frequency band in the vicinity of the frequency f_(LC)according to the first example is employed, impedance in the oscillationfrequency f_(THz) is high, and accordingly, the oscillation condition issatisfied. However, in a region of a low frequency in a range whichincludes the frequency f_(LC) and which is equal to or larger than DCand smaller than the frequency f_(THz), the line 208 has low impedancerelative to the RTD 201, and accordingly, the oscillation condition inExpression (1) is not satisfied. Accordingly, with the configuration ofthe first example, the parasitic oscillation including the LC resonanceis suppressed.

As described above, an oscillation circuit is realized such that theoscillator 200 has high impedance in the desired oscillation frequencyf_(THz) in the terahertz band and has low impedance in the frequencydomain of the parasitic oscillation which includes the LC resonance ofthe frequency f_(LC) and which is equal to or larger than DC and smallerthan the frequency f_(THz). Note that the terms “high impedance” and“low impedance” here mean high impedance and low impedance relative tothe RTD 201 (that is, using the RTD 201 as a reference). Accordingly,even oscillators employing a microstrip resonator may suppresslow-frequency parasitic oscillation caused by a bias circuit or a powersupply structure and stably oscillate terahertz waves of a desiredoscillation frequency f_(THz) defined by a resonator.

With this configuration, loss in the oscillation frequency f_(THz) ofterahertz waves may be reduced by arbitrarily designing the microstripresonator and a size and a material of the structure of the distributedconstant line. Furthermore, appropriate designing may minimize loss ofterahertz waves in the oscillation frequency f_(THz) and maximize lossin a frequency of the parasitic oscillation.

The oscillators of this embodiment and the first example may reduceparasitic oscillation caused by a line structure which is a problem inmicrostrip resonators including a patch antenna. Specifically,additional generation of parasitic oscillation caused by inductance of astrip conductor used to perform bias power supply to the differentialnegative resistance element is reduced. Accordingly, parasiticoscillation may be reduced or suppressed in a frequency domain equal toor larger than DC and smaller than the frequency f_(THz) even in themicrostrip resonators, and accordingly, terahertz waves of the desiredoscillation frequency f_(THz) of the resonators may be more stablyobtained.

Furthermore, since terahertz waves of the oscillation frequency f_(THz)are stably obtained, terahertz waves of the desired oscillationfrequency f_(THz) in the microstrip resonators may be output at highpower.

First Example

A configuration of the oscillator 200 which is an element oscillatingterahertz waves of this example will be described. FIG. 3A is aperspective view of the oscillator 200. The oscillator 200 is an elementwhich oscillates an oscillation frequency f_(THz) of 0.45 THz. In thisexample, a resonant tunneling diode (RTD) is used as the differentialnegative resistance element 201. The RTD 201 of this example has amultiple quantum well structure of InGaAs/InAlAs and InGaAs/AlAs on anInP substrate 230 and an electrical contact layer formed by n-InGaAs.

As the multiple quantum well structure, a triple-barrier structure isused. Specifically, a semiconductor multiple layer structure isconfigured by AlAs (approximately 1.3 nm)/InGaAs (approximately 7.6nm)/InAlAs (approximately 2.6 nm)/InGaAs (approximately 5.6 nm)/AlAs(approximately 1.3 nm). In this structure, InGaAs corresponds to a welllayer and lattice matching InAlAs and lattice mismatching AlAscorrespond to a barrier layer. These layers are undoped layers on whichcarrier dope is intentionally not performed.

Such a multiple quantum well structure is sandwiched by electric contactlayers having an electronic density of 2×10¹⁸ cm⁻³ and formed byn-InGaAs. In a current/voltage I(V) characteristic of a structurebetween such electric contact layers, a peak current density is 280kA/cm², and a range from approximately 0.7 V to approximately 0.9 Vcorresponds to the differential negative resistance region. When the RTD201 has a mesa structure having a diameter of approximately 2 μm, a peakcurrent of 10 mA and a differential negative resistance of approximately−20Ωare obtained.

A resonance unit 202 includes a resonator having a patch conductor 203,a grounding conductor 204, and a first dielectric body 205 a and the RTD201. The resonator includes a square patch antenna having the patchconductor 203 having sides of 200 μm. A BCB (benzocyclobutene, DowChemical Company, ∈r=2.4) having a thickness of approximately 3 μm and asilicon nitride having a thickness of approximately 0.1 μm aresandwiched between the patch conductor 203 and the grounding conductor204 as a first dielectric body 205 a.

The RTD 201 having a diameter of 2 μm is connected between the patchconductor 203 and the grounding conductor 204. The RTD 201 is located ina position shifted from a center of gravity of the patch conductor 203by approximately 80 μm in a resonance direction. Although solo resonancefrequency of the patch antenna is approximately 0.48 THz, an oscillationfrequency (resonance frequency) f_(THz) of the oscillator 200 isapproximately 0.45 THz taking reactance of the RTD which is the RTD 201into consideration.

The patch conductor 203 is connected to a strip conductor 206 of amicrostrip line of the line 208 through a conductor 207. Accordingly,the patch antenna is connected to an MIM capacitance 209 through theline 208. With this configuration, the line 208 is used to connect abias circuit 220 and the resonance unit 202 to each other.

The MIM capacitance 209 is 100 pF in this example. Lines 211 having wirebonding are connected to the MIM capacitance 209, and a power source 212controls a bias voltage of the differential negative resistance element201. A frequency f_(LC) of an LC resonance formed by an inductance L₁ ofthe strip conductor 206 of the microstrip line 208 and the capacitance Cof the resonance unit 202 is approximately 0.08 THz.

The line 208 includes the strip conductor 206, the grounding conductor204, and a second dielectric body 205 b disposed between the stripconductor 206 and the grounding conductor 204. The second dielectricbody 205 b is formed by silicon nitride having a thickness ofapproximately 0.1 μm. Impedance of the line 208 is smaller than anabsolute value of a differential negative resistance of the RTD 201 inthe frequency f_(LC) of the LC resonance. FIG. 3B is a top view of theline 208. As a concrete size of the microstrip line of the line 208, aline having a width a of approximately 6 μm and a length b ofapproximately 100 μm extends from a portion connected to the resonanceunit 202 and a line having a width c of approximately 20 μm and anentire length of approximately 600 μm further extends. The line havingthe width c of approximately 20 μm is connected to the MIM capacitance209. The line having the width c of approximately 20 μm extends in adirection opposite to the MIM capacitance 209 by a length e, bends at anapproximately right angle twice, and extends toward the MIM capacitance209 by the length e. The length e is approximately 200 μm and the lengthd is approximately 400 μm. Furthermore, a distance f between themicrostrip line having the length e and the microstrip line having thelength d is approximately 60 μm.

The patch conductor 203 is connected to the conductor 207 at a node in ahigh-frequency electric field stably existing in the resonance unit 202in an oscillation frequency f_(THz) (=0.45 THz), and suppressesinterference between the line 208 and a resonance electric field ofterahertz waves in the oscillation frequency f_(THz).

With this configuration, as is apparent from the result of theadmittance analysis illustrated in FIG. 2, although the oscillationcondition of Expression (1) is satisfied in the oscillation frequencyf_(THz), Expression (1) is not satisfied in a low frequency domain whichincludes the frequency f_(LC) and which is equal to or larger than DCand smaller than f_(THz) due to low impedance. Accordingly,low-frequency parasitic oscillation caused by the bias circuit and apower supply structure is suppressed in the oscillator 200, and theoscillator 200 may stably oscillate terahertz waves of the desiredoscillation frequency f_(THz) defined by the resonance unit 202including the RTD 201 and the patch antenna.

The oscillator 200 is fabricated as follows. First, on the InP substrate230, layers described below are newly developed by epitaxial growth bymeans of a molecular beam epitaxy (MBE) method or an organometallicvapor-phase epitaxy (MOVPE) method. Specifically, the resonant tunnelingdiode (RTD) 201 formed by n-InP/n-InGaAs and InGaAs/InAlAs in this orderis subjected to epitaxial growth. When an n-type conductive substrate isselected as the InP substrate 230, the epitaxial growth is started fromn-InGaAs.

Subsequently, the RTD 201 is etched in a mesa shape having an arch shapehaving a diameter of approximately 2 μm. As the etching, dry etchingusing EB (electric beam) lithography and ICP (inductive coupling plasma)is used. Alternatively, photolithography may be used. Thereafter, thegrounding conductor 204 is formed on an etched surface by a lift-offprocess. A silicon nitride film having a thickness of approximately 0.1μm is formed on the entire surface as a side wall protective film of thesecond dielectric body 205 b and the resonant tunneling diode.Furthermore, embedding is performed using BCB which is the firstdielectric body 205 a by means of a spin coat method and the dryetching, and the patch conductor 203 of Ti/Pd/Au is formed by thelift-off method.

Next, the BCB outside the patch antenna is removed by the dry etchingmethod so that the silicon nitride film having the thickness of 0.1 μmcorresponding to the second dielectric body 205 b is exposed. Electrodesare formed on upper portions of the conductor 207, the strip conductor206, and the MIM capacitance 209 by the lift-off method. Finally, a Bipattern is formed on a portion corresponding to a resistance 210 by thelift-off method, the grounding conductor 204 and an upper electrode ofthe MIM capacitance 209 are connected to each other, and the line 211and the power source 212 are connected to each other by wire bonding. Inthis way, the oscillator 200 of this example is completed. Electricpower is supplied from the bias circuit 220 to the oscillator 200.Normally, when a bias current is supplied by applying a bias voltage inthe differential negative resistance region, the oscillator 200operates.

In this example, the triple-barrier resonant tunneling diode formed byInGaAs/InAlAs and InGaAs/AlAs grown on the InP substrate has beendescribed as the RTD 201. However, a structure and a material system ofthe RTD 201 are not limited to these, and the element of the presentinvention may be provided employing other structures and othercombinations of materials. For example, a resonant tunneling diodehaving a double barrier quantum well structure or a resonant tunnelingdiode having multiple barrier quantum well structure, such as aquadruple barrier quantum well structure, may be employed.

As a material of such a resonant tunneling diode, one of the followingcombinations may be employed.

-   -   GaAs/AlGaAs, GaAs/AlAs, and InGaAs/GaAs/AlAs formed on a GaAs        substrate    -   InGaAs/AlGaAsSb formed on an InP substrate    -   InAs/AlAsSb and InAs/AlSb formed on an InAs substrate    -   SiGe/SiGe formed on an Si substrate

The structures and the materials described above are appropriatelyselected depending on a desired frequency, for example.

As described above, in the oscillator of this example, the oscillationcircuit which has high impedance in the vicinity of the desiredoscillation frequency f_(THz) in a terahertz waveband and has lowimpedance in a frequency domain of parasitic oscillation which includesthe LC resonance of the frequency f_(LC) and which is equal to or largerthan DC and smaller than the frequency f_(THz) is realized. Accordingly,the oscillator of this example using the microstrip resonator maysuppress low-frequency parasitic oscillation caused by a line structureof the bias circuit, the power supply structure, and the like and stablyoscillate terahertz waves of the desired oscillation frequency f_(THz).

Furthermore, since the stable terahertz waves of the oscillationfrequency f_(THz) may be obtained, terahertz waves of the desiredoscillation frequency f_(THz) in the microstrip resonator may be outputat higher power.

Second Example

In a second example, elements of first to third modifications of theoscillator 200 according to the first example will be described. First,a structure of an oscillator 300 serving as an element of the firstmodification will be described with reference to FIGS. 4A and 4B. FIG.4A is a perspective view of the element 300, and FIG. 4B is a sectionalview of the element 300 taken along a line IVB-IVB of FIG. 4A. As withthe oscillator 200, the oscillator 300 is an element which oscillates anoscillation frequency f_(THz) of 0.45 THz. A structure of a resonanceunit 302 of the oscillator 300 is different from that of the resonanceunit 202 of the oscillator 200. Furthermore, a structure of a line 308of the oscillator 300 is also different from that of the line 208 of theoscillator 200 according to the first example. The structure of the lineis designed where appropriate. Other configurations are the same asthose of the oscillator 200, and detailed descriptions thereof areomitted.

The resonance unit 302 includes the patch conductor 203, the groundingconductor 204, and a first dielectric body 305 a disposed between thepatch conductor 203 and the grounding conductor 204. A portion of a sidewall of the first dielectric body 305 a is not substantially orthogonalto the substrate 230, and an angle defined by the substrate 230 and thefirst dielectric body 305 a is approximately 60 degrees. The patchconductor 203 is connected to the strip conductor 206 of the microstripline serving as the line 208 through the fifth conductor 207.

The fifth conductor 207 is disposed on a slope which is tapered suchthat an angle defined by a portion of the first dielectric body 205 aand the substrate 230 is approximately 60 degrees. The fifth conductor207 is a plug used to cover a gap (difference in height) between thepatch conductor 203 and the strip conductor 206. This configuration mayreduce increase of resistance caused by defect of coverage of metal in agap portion between the first and second dielectric bodies 305 a and 205b which have different thicknesses.

An oscillator 400 of the second modification of the oscillator 200 willbe described. FIG. 5A is a perspective view of the oscillator 400, FIG.5B is a sectional view of the oscillator 400 taken along a line VB-VB ofFIG. 5A, and FIG. 5C is a sectional view of the oscillator 400 takenalong a line VC-VC of FIG. 5A. The oscillator 400 which oscillates theoscillation frequency f_(THz) of 0.45 THz uses a strip line serving as adistributed constant line of a line 408 used to connect the a resonanceunit 402 and a bias circuit to each other. Other configurations are thesame as those of the oscillator 200, and only important portions aredescribed.

The line 408 of the oscillator 400 includes a strip conductor (thirdconductor) 406, a second dielectric body 405 b, and a third dielectricbody 422. The strip conductor 406 is embedded in a dielectric layerincluding the third dielectric body 422 having a thickness of t₁ and thesecond dielectric body 405 b having a thickness of t₂. Furthermore, thedielectric layer is sandwiched between a sixth conductor 421 and thesecond conductor (fourth conductor) 204 in a vertical direction. Thestrip conductor 406 and the patch conductor 203 are electricallyconnected to each other through the fifth conductor 207.

The fifth conductor 207 is a plug used to cover a gap (difference inheight) between the patch conductor 203 and the strip conductor 406, andthe resonance unit 202 and the strip line 408 are coupled with eachother by DC coupling. The sixth conductor 421 and the groundingconductor 204 may be electrically and physically connected to eachother.

An admittance characteristic of the oscillator 400 will now be describedwith reference to FIG. 6. FIG. 6 is a graph illustrating a result ofanalysis of the admittance characteristic of the oscillator 400 andplotting a real part Re[Y_(RTD)] of admittance of an RTD 401 which is adifferential negative resistance element and a real part Re[Y_(ANT)] ofadmittance of the patch antenna. In the graph, as for the real partRe[Y_(ANT)] of the admittance of the antenna, results of analysis in twocases, that is, a case where a thickness of the dielectric layerincluding the third dielectric body 422 and the second dielectric body405 b satisfies an expression “t1=t2=100 nm” and a case where thethickness satisfies an expression “t1=t2=500 nm”, are illustrated.Furthermore, as for a gain of the RTD 401, that is, the real partRe[Y_(RTD)] of the admittance, results of analysis in two cases, thatis, a case where a diameter of the RTD 401 is 2 μm and a case where thediameter is 3 μm, are illustrated.

According to the graph, the impedance in a frequency band in thevicinity of the frequency f_(LC) may be controlled by controlling athickness of the dielectric layer (the third dielectric body 422 and thesecond dielectric body 405 b) of the line 408. Note that the impedancein the frequency band in the vicinity of the frequency f_(LC) may becontrolled not only by controlling the thickness of the dielectric layerbut also by selecting a length of the line 408 and materials of thesecond dielectric body 405 b and the third dielectric body 422, and theselection is appropriately performed in accordance of the gain of theRTD 401. With the configuration of the oscillator 400, oscillation issuppressed in a low frequency domain equal to or larger than DC andsmaller than the frequency f_(THz) due to loss whereas the oscillationcondition may be satisfied in the desired oscillation frequency f_(THz)since the loss is small.

An oscillator 500 of the third modification of the first example will bedescribed. FIG. 7A is a perspective view of the oscillator 500, FIG. 7Bis a sectional view of the oscillator 500 taken along a line VIIB-VIIBof FIG. 7A, and FIG. 7C is a sectional view of the oscillator 400 takenalong a line VIIC-VIIC of FIG. 7A. The oscillator 500 is an elementwhich oscillates an oscillation frequency f_(THz) of 0.45 THz. Theoscillator 500 uses a rectangular coaxial line as a line 508 whichconnects a resonance unit 502 and a bias circuit to each other. Otherconfigurations are the same as those of the oscillator 200, andtherefore, detailed descriptions thereof are omitted and only importantportions are described hereinafter.

The rectangular coaxial line of the line 508 is a distributed constantline. The line 508 includes a strip conductor (third conductor) 506, asixth conductor 521, a third dielectric body 522, and a fourth conductor(grounding conductor) 204. The strip conductor 506 is embedded in adielectric layer including the third dielectric body 522 having athickness of h₁ and the second dielectric body 505 b having a thicknessof h₂, and the dielectric layer is surrounded by a conductive layerincluding the sixth conductor 521 and the fourth conductor 204. Thestrip conductor 506 and the patch conductor 203 are electricallyconnected to each other through the fifth conductor 207 which is a plugcovering a gap (difference in height). The line 508 is a low impedanceline using the absolute value of the differential negative resistance ofthe element 201 as a reference.

An admittance characteristic of the oscillator 500 will now be describedwith reference to FIG. 8. FIG. 8 is a graph illustrating a result ofanalysis of the admittance characteristic of the oscillator 500 andplotting a real part Re[Y_(RTD)] of admittance of the RTD correspondingto the element 201 and a real part Re[Y_(ANT)] of admittance of thepatch antenna. In the graph, as for the real part Re[Y_(ANT)] of theadmittance of the antenna, results of analysis in two cases, that is, acase where a thickness of the dielectric layer including the thirddielectric body 522 and the second dielectric body 205 b satisfies anexpression “h₁=h₂=100 nm” and a case where the thickness satisfies anexpression “h₁=h₂=500 nm”, are illustrated. Furthermore, as for a gainof the RTD, that is, the real part Re[Y_(RTD)] of the admittance,results of analysis in two cases, that is, a case where a diameter ofthe RTD is 2 μm and a case where the diameter is 3 μm, are illustrated.

According to the graph, the impedance in a frequency band in thevicinity of the frequency f_(LC) may be controlled by controlling thethicknesses of the third dielectric body 522 and the second dielectricbody 205 b of the strip line 508. Note that the impedance in thefrequency band in the vicinity of the frequency f_(LC) may be controllednot only by controlling the thickness of the dielectric layer but alsoby selecting a length of the line 508 or materials of the seconddielectric body 205 b and the third dielectric body 522, and theselection is appropriately performed in accordance of the gain of theelement 201. With the configuration of the oscillator 500, oscillationis suppressed in a low frequency domain equal to or larger than DC andsmaller than the frequency f_(THz) due to loss whereas the oscillationcondition may be satisfied in the desired oscillation frequency f_(THz)since the loss is small.

The first to third modifications of the oscillator 200 have beendescribed hereinabove. Also in the first to third modificationsdescribed above, since the line which connects the resonance unit andthe bias circuit to each other becomes a low impedance circuit using theabsolute value of the differential negative resistance of thedifferential negative resistance element as a reference, parasiticoscillation caused by a line structure may be reduced. Consequently,terahertz waves in the desired oscillation frequency f_(THz) may bestably oscillated.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

Although it is assumed that a carrier is an electron in the embodimentand the examples described above, for example, the present invention isnot limited to this and a hole may be employed. Furthermore, materialsof substrates and dielectric bodies are selected depending on theintended use, and a semiconductor, such as silicon, gallium arsenide,indium arsenide, and gallium phosphide, glass, ceramic, Teflon(registered trademark), and resin, such as polyethylene terephthalate,may be used.

Furthermore, although a square patch is used as a resonator forterahertz waves in the embodiment and the examples described above, ashape of the resonator is not limited to this and a resonator having apatch conductor of a polygonal shape, such as a rectangular shape or atriangular shape, circle, or an oval, may be used.

Furthermore, the number of differential negative resistance elementsintegrated in an oscillator is not limited to one, and a resonator mayinclude a plurality of differential negative resistance elements.Moreover, the number of lines is also not limited to one, and aplurality of lines may be provided.

The oscillators described in the embodiment and the examples above maybe used as detectors which detect terahertz waves. Furthermore,terahertz waves may be oscillated and detected using the oscillatorsdescribed in the embodiment and the examples above.

This application claims the benefit of Japanese Patent Application No.2014-039031, filed Feb. 28, 2014 and No. 2015-020635, filed Feb. 4,2015, which are hereby incorporated by reference herein in theirentirety.

The invention claimed is:
 1. An element which oscillates or detectsterahertz waves, the element comprising: a resonance unit including adifferential negative resistance element, a first conductor, a secondconductor, and a first dielectric body; a bias circuit configured tosupply a bias voltage to the differential negative resistance element,the bias circuit including a resistance and capacitance, each of theresistance and the capacitance being electrically connected in parallelwith the differential negative resistance element; and a line configuredto electrically connect the resonance unit and the bias circuit to eachother, wherein the differential negative resistance element and thefirst dielectric body are disposed between the first and secondconductors, the line includes a third conductor, a fourth conductor, asecond dielectric body disposed between the third and fourth conductors,and a fifth conductor which electrically connects the first and thirdconductors to each other, a thickness of the second dielectric body issmaller than a thickness of the first dielectric body, and a width ofthe third conductor is smaller than a width of the first conductor. 2.The element according to claim 1, wherein a real part of an impedance ofthe line in a frequency of resonance caused by inductance of the lineand capacitance of the resonance unit is equal to or smaller than avalue ten times as large as the absolute value of the differentialnegative resistance of the differential negative resistance elementusing the differential negative resistance of the differential negativeresistance element as a reference.
 3. The element according to claim 2,wherein the real part of the impedance of the line in the frequency isequal to or smaller than the absolute value of the differential negativeresistance of the differential negative resistance element using thedifferential negative resistance of the differential negative resistanceelement as a reference.
 4. The element according to claim 1, whereindielectric loss of the line in a frequency of resonance caused byinductance of the line and capacitance of the resonance unit is equal toor larger than one-tenth of an absolute value of an inverse number ofthe differential negative resistance of the differential negativeresistance element.
 5. The element according to claim 4, wherein thedielectric loss of the line in the frequency is equal to or larger thanan absolute value of an inverse number of the differential negativeresistance of the differential negative resistance element.
 6. Theelement according to claim 1, wherein a characteristic impedance of theline in a frequency of resonance caused by inductance of the line andcapacitance of the resonance unit is equal to or smaller than a valueten times as large as the absolute value of the differential negativeresistance of the differential negative resistance element.
 7. Theelement according to claim 6, wherein the characteristic impedance ofthe line in the frequency is equal to or smaller than the absolute valueof the differential negative resistance of the differential negativeresistance element.
 8. The element according to claim 1 which satisfies,when “Re[Y_(RTD)]” denotes a real part of admittance of the differentialnegative resistance element and “Re[Y_(ANT)]” denotes a real part ofadmittance of a resonator including the first and second conductors andthe first dielectric body, an expression “Re[Y_(RTD)]+Re[Y_(ANT)]≤0” ina resonance frequency of the oscillated or detected terahertz waves, andan expression “Re[Y_(RTD)]+Re[Y_(ANT)]>0” in the frequency of resonancecaused by the inductance of the line and the capacitance of theresonance unit.
 9. An element which oscillates or detects terahertzwaves, the element comprising: a resonance unit including a differentialnegative resistance element, a first conductor, a second conductor, anda first dielectric body; a bias circuit configured to supply a biasvoltage to the differential negative resistance element, the biascircuit including a resistance and capacitance, each of the resistanceand the capacitance being electrically connected in parallel with thedifferential negative resistance element; and a line configured toelectrically connect the resonance unit and the bias circuit to eachother, wherein the differential negative resistance element and thefirst dielectric body are disposed between the first and secondconductors, the line includes a third conductor, a fourth conductor, asecond dielectric body disposed between the third and fourth conductors,and a fifth conductor which electrically connects the first and thirdconductors to each other, a thickness of the second dielectric body issmaller than a thickness of the first dielectric body, and the thicknessof the second dielectric body is equal to or larger than 0.001 μm andequal to or smaller than 1μm.
 10. An element which oscillates or detectsterahertz waves, the element comprising: a resonance unit including adifferential negative resistance element, a first conductor, a secondconductor, and a first dielectric body; a bias circuit configured tosupply a bias voltage to the differential negative resistance element,the bias circuit including a resistance and capacitance, each of theresistance and the capacitance being electrically connected in parallelwith the differential negative resistance element; and a line configuredto electrically connect the resonance unit and the bias circuit to eachother, wherein the differential negative resistance element and thefirst dielectric body are disposed between the first and secondconductors, the line includes a third conductor, a fourth conductor, asecond dielectric body disposed between the third and fourth conductors,and a fifth conductor which electrically connects the first and thirdconductors to each other, a thickness of the second dielectric body issmaller than a thickness of the first dielectric body, and the line is alow impedance line in a frequency of resonance caused by inductance ofthe line and capacitance of the resonance unit using an absolute valueof a differential negative resistance of the differential negativeresistance element as a reference.
 11. The element according to claim 1,wherein, when “d₁” denotes a length of the line and “d₂” denotes alength from the differential negative resistance element to theresistance, a frequency of resonance caused by inductance of the lineand capacitance of the resonance unit is included in a frequency bandcorresponding to a wavelength band of 4×d₁ or more and 4×d₂ or less. 12.The element according to claim 1, wherein the fifth conductor physicallyconnects to the first and third conductors.
 13. The element according toclaim 12, wherein a width of the fifth conductor is smaller than a widthof the first conductor.
 14. The element according to claim 13, whereinthe width of the fifth conductor is equal to or smaller than one-tenthof a wavelength of the terahertz waves.
 15. The element according toclaim 1, wherein the width of the third conductor is equal to or smallerthan one-tenth of a wavelength of the terahertz waves.